1. Field of the Invention
The present invention generally relates to photovoltaic cell interconnects and, more particularly, to an improved photovoltaic cell interconnect that provides increased stress relief without an increase in solar panel shading.
2. Description of Related Art
The interest in photovoltaic (PV) cells, including large area cells that are greater than about 2 cm by 2 cm, continues as concerns over pollution and limited resources continue. The continued interest has been in both terrestrial and non-terrestrial applications. In the non-terrestrial environment of outer space, the concern over limited resources of any type is a major one. This is because the need to increase the amount of a resource increases the payload, and an increased payload can increase the cost of a launch more than linearly. However, with the ready availability of solar energy in outer space for a spacecraft such as a satellite, the conversion of solar energy into electrical energy is an obvious alternative to increased payload.
Irrespective of the application, and as with any energy generation system, efforts have been ongoing into increasing the output and/or efficiency of PV cells. In terms of output, multiple cells or layers having different energy bandgaps have been stacked so that each cell or layer can absorb a different part of the wide energy distribution in the sunlight. A number of the stacked arrangements or cells have been provided in the form of an array on a substrate.
The efficiency of the array of solar cells is limited by the requirement of the cells being electrically connected to one another in series, in parallel or both. This has been accomplished by the use of interconnects between immediately adjacent cells. However, the individual solar cells and their substrate can be subject to significant mechanical vibration and/or thermal cycling, particularly in outerspace. The thermal cycling, however, leads to thermal expansion and contraction of the distance separating the cells, thereby causing stress on the interconnects. With greater stress in terms of frequency and magnitude, there can be a shorter life expectancy of the interconnect. With a shorter life expectancy of the interconnect, a solar panel in which the interconnect is utilized will have a shorter life expectancy. Ultimately, the spacecraft on which the solar panel is used will have a shorter life and result in greater costs.
Various past designs have attempted to alleviate the stress factor of the interconnect. For example, U.S. Pat. No. 5,006,179 discloses a solar cell interconnect that includes a planar, central portion that is positioned parallel to the edges of opposing solar cells. Two connecting parts are fixed at opposite ends of the central portion. One connecting part has an L-shaped configuration that is soldered to an edge of one solar cell. The opposite connecting part is also L-shaped but has a closed loop portion that is soldered to a planar surface of the adjacent solar cell. A disadvantage, however, to such design is the complexity in manufacturing.
Also shown in U.S. Pat. No. 5,006,179 is a prior art interconnect having a single open loop disposed between two planar ends. This design, however, was considered as being subject to fracturing of the interconnect and cell itself under repeated stress. There was also some suggestion that the prior art design prevented the cells in an array from being close to one another, thus increasing the space requirements.
U.S. Pat. No. 4,193,820 shows a solar cell interconnect having an overall Z-shape. Two flat strips are disposed at opposite ends. A hinge element is between the opposite ends. The hinge element is generally U-shaped and extends longitudinally between the edges of adjacent cells. A disadvantage to this design is that the flat strips are apparently covered by an adhesive that bonds together the cell and the substrate, thus causing the flexing members to seize and fail when the adhesive, in which the members are imbedded, is subjected to the low temperatures associated with satellite orbits.
Also shown in U.S. Pat. No. 4,193,820 is a prior art interconnect which is of a Z-shape. However, the central or middle portion does not have a U-shaped configuration. This prior art design was regarded as being unable to withstand severe strains in a transverse direction to the edges of adjacent cells.
U.S. Pat. No. 3,819,417 discloses mechanical, non-conducting solar cell interconnect strips. The non-conducting strips are interwoven. Thereby, if one strip goes over one cell and under the next, the other strip goes under the first cell and over the next. The portion of the strips that are positioned intermediate the edges of adjacent cells have a single open loop configuration. Electrical connection between cells is provided by regions of metallic patterns placed on the non-conducting strips. A disadvantage, however, to such mechanical and electrical interconnect is that two types of connections are required from two different materials, which tends to complicate manufacturing. Another disadvantage is that the interconnects take up active areas of the tops of the cells.
Further shown in U.S. Pat. No. 3,819,417 is a prior art interconnect that is generally Z- or S-shaped. Such prior art design was considered to provide insufficient flexibility. This was thought to lead to failure at the solar cell-interconnect interface.
As can be seen, there is a need for an improved solar cell interconnect that has a longer life and can withstand greater thermal expansions and/or mechanical vibrations. An improved solar cell interconnect is needed which is low cost, simple to manufacture, and easy to utilize. Also needed is an interconnect that is adaptable to different manners of connection. A further need is for a solar cell interconnect which minimizes the amount of shading of the cell.